Gas separation through a membrane is useful in various fields because it does not require much energy for the separation compared with other methods. Conventionally, as such a gas separating membrane, a cellulose acetate Membrane is well known. The cellulose acetate membrane, however, has poor chemical resistance and heat resistance so that it does not always have sufficient performance in practical applications. As a separating membrane having higher heat resistance, a polysulfone semi-permeable membrane has been produced on an industrial scale, which however is not sufficient in permeating performance. As a permselective membrane for oxygen, a silicone membrane is known but it has not yet attained a sufficient mechanical strength so that its products satisfactory from the industrial viewpoint have not been available yet. Recently, studies on a separating membrane obtained using a polyimide resin excellent in not only strength and heat resistance but also gas selective permeability has been made intensively. Such a gas separating membrane or composite membrane made of a polyimide resin is also insufficient for practical use from the viewpoints of permeable speed and selective separativity.
As another heat resistant resin, a polycarbodiimide resin synthesized from an organic diisocyanate is known. This resin becomes insoluble or infusible upon preparation and its molecular weight cannot be controlled easily, which makes it difficult to prepare a homogeneous and tough membrane. A practically usable gas separating membrane has not therefore been available now from such a resin.